1. Field
The following description relates to an optical recording/reproducing apparatus, and more particularly, to a compatible near field optical recording/reproducing apparatus that is used in both near field and far field recording/reproduction.
2. Description of the Related Art
The size of an optical spot is an important factor affecting the storage capacity of information storage media, such as optical disks. Information corresponding to marks or pits having small sizes may be recorded and/or reproduced with small optical spots. Thus, the storage density may be increased if the size of the optical spots is small. A wavelength of light may be reduced or a numerical aperture (NA) of an objective lens may be increased in order to reduce the size of an optical spot. Compact discs (CDs), digital versatile discs (DVDs), high definition DVDs (HD DVDs), and Blu-ray discs (BDs) have been developed having different wavelengths of light and NAs in order to reduce the size of an optical spot. Recording/reproducing for CDs, DVDs, BDs, HD DVDs is performed using a far field recording/reproducing technique, which is a technique in which the distance between an objective lens and an information storage medium is on the order of millimeters, which corresponds to a far field having a dimension thousands of times greater than a wavelength of light.
In a near field optical recording/reproduction technique that has been recently studied, the distance between a lens and an information storage medium is of the order of tens of nanometers, which is shorter than a wavelength of light. The near field optical recording/reproduction technique is also referred to as a “near field technique.” Even if such a near field optical recording/reproduction technique is performed using the same wavelength of light that is used in a far field optical recording/reproduction technique, an NA may be greater than 1. Therefore, the size of an optical spot may further be reduced to further increase data density.
It is desirable for an optical recording/reproducing apparatus to have backward compatibility so that recording/reproduction may be performed using another information storage medium. Thus, it is desirable for the near field recording/reproducing (NFR) technique to have backward compatibility.
Far field recording/reproducing (FFR) techniques and NFR techniques are different from each other in terms of compatibility. For example, the techniques differ in NA, working distance between information storage media, and thickness of a cover layer of the information storage medium. According to the NFR technique, a focal point is formed at a region close to the bottom surface of a ball lens, and according to the FFR technique, a focal point is formed beyond the working distance in order to prevent a collision between an objective lens and an information storage medium and a cover layer of about 100 μm. In order to satisfy geometric optical characteristics of the two optical systems, the incident angle of a beam incident onto a focusing lens may be significantly changed.